The present invention relates to a positioning ring for a hydraulically operated warning device, in particular a positioning ring for a combination proportioning valve and warning switch assembly.
Automatic vehicle proportioning valves typically utilize a differential area piston or sleeve in the secondary brake circuit so that when a predetermined pressure level is reached, the piston or sleeve shifts in the direction of the smaller area piston end. As the piston shifts, the piston approaches a poppet or seal and this establishes a pressure restriction. Upon a further increase in secondary pressure communicated to the proportioning valve piston or sleeve, the outlet pressure will rise at a rate determined by the differential areas at the ends of the piston. Copending U.S. patent application No. 266,964 now U.S. Pat. No. 4,929,033 entitled "Primary and Secondary Pressure-Balanced Proportioning Valve", incorporated by reference herein, discloses a two-part piston assembly wherein one piston part includes an extension received within an interior bore of the other piston or sleeve part. The other piston or sleeve part includes a recess thereabout which is engaged by the probe of a warning switch. Should there exist a failure of pressure in either the secondary or primary pressure chambers of the associated master cylinder, the other piston or sleeve part will be displaced and cause the probe part to be retracted so that a pressure failure warning is provided to the vehicle operator. U.S. Pat. No. 4,929,033 illustrates such an assembly wherein the other piston or sleeve part includes thereabout a square-shaped centering ring which abuts a radial shoulder of the other piston part and a seal abuts the ring. Forces acting upon the seal, ring, and sleeve part as a result of pressure in the secondary chamber tend to be greater than the counteracting forces acting upon the opposite end of the other piston or sleeve part and generated by pressure in the primary chamber. This permits the other piston or sleeve to be moved axially until the ring engages a radial portion or shoulder of the housing which extends into the bore. Currently produced pressure differential switches utilize a timed pressure-leakage method in order to detect a missing or reversed centering ring/sealing ring installation. In other words, if the seal or centering ring are misassembled or missing from the assembly, this is detected by a pressure-leakage method. This pressure-leakage method utilizes an undercut in the other piston or sleeve part which generally permits leakage past the seal if incorrect installation occurs. Although testing has indicated that the other piston part or sleeve does cause the detection of a missing or reversed seal or centering ring by means of the pressure-leakage method, it is theoretically possible that detection may not occur due to the possible occurrence of the sealing ring sealing or seating at the aligned sleeve/housing body interface. In order to prevent a possible failure to detect either the misassembly or inclusion of the seal and centering ring, it is desirable to provide a centering ring which if missing or misassembled would cause movement of the other piston or sleeve part so that the warning switch is activated and an electrical signal from the actuated warning switch provides the desired detection, rather than utilizing the measured time/leakage method. This would provide an improved method of fault detection which better assures that correct assembly of the porportioning valve/warning switch combination has been accomplished.